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Sharpening >

The name is a carryover from a technique used with negatives to increase apparent sharpness.

A negative is sandwiched with a slightly out-of-focus and underexposed positive of the negative.

This positive is called an unsharp mask.

Because the mask is out-of-focus, the image on it is slightly larger than the image on the negative.

When printed, this creates a border around the edges of subject matter in the negative.

Negative

Unsharp Mask

Sandwich

The border is the opposite density (shade) of the edge in a negative.

Therefore, more contrast is created on the edges, which we see as increased sharpness.

This is similar to what the Unsharp Mask filter does in Photoshop Elements.